In order to reduce transportation costs and the packaging volume of products, most of the manufacturers tend to provide conventional box-structure furniture with a detachable multi-piece structure. This type of detachable box structure has many components. The installation needs to be done by referring to the instructions. In addition, the assistance from others is required to support the positioning, so the assembly is not only difficult but also time-consuming. Generally, it may take about 10-20 minutes to complete the assembly. Moreover, the structure of the assembled box lacks stability. For this reason, an easy-to-fold and easy-to-assemble box and a pet house using the box are provided.